1. Field of the Invention
The present invention relates to a semiconductor device which has a TFT (thin-film transistor) built onto an insulating substrate made of glass or the like, and to a method for its preparation.
2. Description of the Prior Art
Thin-film transistors (hereunder, TFTs) are known which employ thin-film semiconductors. These TFTs are constructed by forming a thin-film semiconductor on a substrate and using the thin-film semiconductor. These TFTs are used in various integrated circuits, but particular attention is being given to their use in electrooptical devices, especially as switching elements constructed for the respective picture elements of active matrix-type liquid crystal displays and driving elements formed in peripheral circuit sections.
The TFTs used in these devices generally employ thin-film silicon semiconductors. Thin-film silicon semiconductors are largely classified into two types: amorphous silicon semiconductors (a-Si) and silicon semiconductors with crystallinity. Amorphous silicon semiconductors have a low preparation temperature, they may be relatively easily prepared by a vapor phase process, and they lend themselves well to mass preparation, for which reasons they are the most widely used type; however, their physical properties such as conductivity, etc. are inferior in comparison with those of silicon semiconductors with crystallinity, and thus ardent attempts to establish new methods of preparing silicon semiconductor TFTs with crystallinity have been made in order to obtain more high-speed properties in the future. As crystalline silicon semiconductors, there are known polycrystalline silicon, microcrystalline silicon, amorphous silicon which also contains crystalline components, and semi-amorphous silicon in an intermediate state between crystalline and amorphous solids.
The following methods are known for obtaining thin-film silicon semiconductors with crystallinity:    (1) Direct formation of a crystalline film at the time of its formation.    (2) Formation of an amorphous semiconductor film to which crystallinity is then imparted using the energy of laser light.    (3) Formation of an amorphous semiconductor film to which a crystallinity is then imparted by adding heat energy.
However, in method (1), it is technically difficult to form a uniform film with satisfactory semiconducting properties over the entire surface of the substrate, while another drawback is the cost, since with the film forming temperature being as high as 600° C. or above inexpensive glass substrates cannot be used. In method (2), there is first the problem of a small irradiation area of laser light, such as that from an excimer laser which is the type most generally used at the present time, resulting in a low throughput, while the stability of the laser is not sufficient for uniform processing of the entire surface of large-surface-area substrates, and thus the method is thought to be a next-generation technique. Method (3) has a relative advantage over methods (1) and (2) in that it is suitable for large surface areas, but it also requires high heating temperatures of 600° C. and above, and thus it is necessary to lower the heating temperature when using low-cost glass substrates. Particularly, in the case of liquid crystal displays presently in use there is a continuous drive toward large-size screens, and consequently the use of large-size glass substrates is also necessary. When large-size glass substrates are employed in this manner, shrinkage and warping which occur during the heating process indispensable to the preparation of the semiconductors result in lower precision of mask alignment, etc., and thus a major problem is inherent. Particularly, in the case of 7059 glass which is presently the most widely used type of glass, the warping point is 593° C. and consequently major deformities are caused with the conventional processes for heat crystallization. In addition to the problem of temperature, the heating time required for crystallization in the existing processes often reaches a few dozen hours or more, and thus a further shortening of this time is necessary.
A greater problem is that fact that, since silicon thin films with crystallinity prepared by these methods depend on coincidental generation of nuclei and crystal growth therefrom, it has been practically impossible to control the particle size, orientation, etc. Many numerous attempts to control these have been made up to the present time, and as an example thereof there may be mentioned the patented invention described in Japanese Patent Application Publication HEI No. 5-71993. Nevertheless, at present, methods such as the one described in this patent publication still use coincidentally generated nuclei within a restricted range, and therefore at present control of the orientation of the film has not been complete, and there has been absolutely no control of the particle size.